The Northern Alberta Tar Sands are considered to be one of the world's largest remaining oil reserves. The tar sands are typically composed of about 70 to about 90 percent by weight mineral solids, including sand and clay, about 1 to about 10 percent by weight water, and a bitumen or oil film, that comprises from trace amounts up to as much as 21 percent by weight. Typically ores containing a lower percentage by weight of bitumen contain a higher percentage by weight of fine mineral solids (“fines”) such as clay and silt.
Unlike conventional oil deposits, the bitumen is extremely viscous and difficult to separate from the water and mineral mixture in which it is found. Generally speaking, the process of separating bitumen from the tar sands comprises six broad stages. 1) Initially, the oil sand is excavated from its location and passed through a crusher or comminutor to comminute the chunks of ore into smaller pieces. 2) The comminuted ore is then typically combined with hot process water to aid in liberating the oil. The combined tar sand and hot water is typically referred to as a “slurry”. Other agents, such as flotation aids may be added to the slurry. 3) The slurry is then passed through a “conditioning” phase in which the slurry is allowed to mix and dwell for a period to create froth in the mixture. The term “conditioning” generally refers to a state whereby the slurry is sufficiently mixed and aerated that a commercially viable amount of the bitumen has left the mineral component to form an oily film over the bubbles in the slurry. 4) Once the slurry has been conditioned, it is typically passed through a series of separators for removing the bitumen froth from the slurry. 5) After the slurry has been sufficiently processed to remove the maximum practical amount of bitumen, the remaining material, commonly known as the “tails”, is typically routed into a tailing pond for separation of the sand and fines from the water. Due to the time required to clarify the tailings water, the process requires the continual addition of fresh water. 6) The separated bitumen and water is then delivered to a secondary extraction process that further removes mineral and water content and provides a diluted bitumen product for delivery to an upgrader that converts the bitumen into a commercially usable product.
It has been recognized for a long time that, since the bitumen comprises a relatively small percentage by weight of the ore initially extracted, separation of the mineral content from the ore as soon as possible after excavation would lead to the most efficient and cost effective mining process. It has also been recognized that it would be useful to immediately recycle the process water used to create the slurry rather than the current requirement of continually using fresh water due to the slow process of clarifying tailings water. While these advantages have been known, to date there has been no commercially viable method of extracting the mineral content soon after excavation and recycling the process water. Generally, the sand and fines settle out of the tails at different rates with the fines taking a long time to settle out. This results in a tailings pond comprised of a sand deposit, a suspension of fines and water, and a thin layer of clarified water on the top of the tailings pond. While the thin layer of clarified water is clean enough that it may be siphoned off and recycled as process water, the bulk of the water remains trapped in the suspension. Furthermore, as settling progresses, the settled fines trap a significant percentage by weight of water. The net result has been extensive tailings ponds that require significant containment structures and associated ongoing maintenance as well as increasing transportation costs as the tails must be transported to new tailings deposition sites as existing ponds are filled. Handling the tails and transporting them to available tailings ponds has become a difficult and expensive logistical problem in mining the oil sands. Additionally, a large volume of water is tied up in existing ponds, necessitating a large ongoing demand for fresh process water.
Over the years, a variety of methods have been used to process and transport the sand from the excavation site. Initially, oil sand excavation and transport were completely mechanical via conveyor belts extending from the mine face to a large facility for processing the mined ore. As mining progressed the conveyors lengths were increased to transport ore from the receding mine face to a large processing facility. The use of conveyors led to many difficulties including high energy costs and mechanical breakdown which led to work stoppage. As mining continued, the use of conveyors to transport the ore over extended distances became unworkable.
Large ore trucks were instituted to replace the conveyor system for transporting ore from the mine face to the processing facility. The ore trucks, however, are expensive to purchase and operate and often create inefficiencies in the production process.
As described in Canadian Patent No. 2,029,795, it was determined that it was preferable to deliver the ore by truck from the mine face to an intermediate site where the ore would be crushed and combined with hot process water at a slurry preparation facility to create a pumpable slurry for transport through a pipe. This “hydro-transport” process served the dual purpose of efficiently transporting the slurry from an intermediate site relatively near the mine face to the large processing facility and allowing time for the slurry to be sufficiently conditioned on route. Provided the hydro-transport was over a sufficiently large enough distance that the dwell time in the pipe was sufficiently long, typically at least 1 kilometer, the slurry would arrive at the processing facility already conditioned and ready for separation. Thus, the previously required separate conditioning step could be omitted from the process.
While the hydro-transport solved some of the difficulties with transporting the ore from the mine site face to the separation facility, it did not solve the long term need to reduce the mechanical transport of large volumes of mined oilsand from the mine face to the intermediate site. As will be appreciated, continual excavation results in the active mine site face being located further and further from the crusher and slurry preparation facility. Solutions to date have typically relied on constructing longer conveyor belts to transport the ore, or use additional trucks, to move the ore from the mine face to the slurry facility at the intermediate site. Though these solutions provide temporary relief, they do not solve the inefficiency of transporting the mineral component further than required.
One concept was to do away with the transport step completely by locating all of the ore processing machinery near the mine face. An example of this concept is disclosed in Canadian Patent No. 2,092,121 and Canadian Patent No. 2,332,207. These references disclose a single mobile excavator and bitumen extraction facility, commonly referred to as a tar sand combine, that follows the mine face as digging progresses. This solution is not ideal as it requires the continuous transport of a large amount of extremely heavy machinery and water including a slurry preparation facility. In addition, connections to the hydro-transport pipeline and process water supply line must be continuously extended as the combine advances. Further, some embodiments suggest separating the mineral component at the mine face. Since the slurry must first be conditioned prior to separation, these embodiments require the continual transport of large volumes of slurry as it is conditioned.
In Canadian Patent Application No. 2,453,697, the idea of a process line comprising a combination of mobile and relocatable equipment units at the face of an oil sand mine site is suggested. The '697 application proposes a process comprising a mobile excavator that advances along a mine face, a mobile comminutor that advances behind the excavator to crush the mined ore to a conveyable size, and a relocatable conveyor that extends along the mine face for receiving the crushed oil sand and conveying it to a relocatable slurry facility for preparing slurry for hydro-transport. The slurry facility may be connected directly to a fixed pipe for hydro-transport. The process line of the '697 application allows for relatively small components, such as the excavator and comminutor, to be mobile and follow the mine face as digging progresses. Less transportable equipment such as the slurry facility and hydro-transport pipe, are relocatable. That is, they are stationed in a fixed location for an extended period of time (months), but may be relocated once the excavator has removed all of the ore within near proximity to the relocatable conveyor.
The disclosure of the '697 application suffers from several limitations. First, the dwell time of the slurry facility is determined solely by the rate of excavation and the length of the first relocatable conveyor. Thus, to increase the dwell time in a particular location, either the rate of excavation must be slowed or the length of the conveyor must be increased. The Northern Alberta region has extremely harsh weather conditions and it has been found that extensive conveyors consume a considerable amount of energy, and are prone to break down resulting in work stoppage. For this reason, the length of the conveyor is preferably not overly long. However, it is also desirable that the slurry facility be relocated as seldom as possible necessitating a minimum length of conveyor in order to access a suitable volume of ore to supply the slurry facility. An additional limitation of the '697 application is that a practical relocatable slurry facility or relocatable desanding facility is not disclosed.
A further problem faced by the industry is the extensive use of water to extract the bitumen from the ore. While the sand portion of the mineral component may be practically removed from the slurry, the fine tailings, clay and other fine-sized material, is difficult to remove from the tailings and tends to remain in suspension. The solution to date has been to store the tailings in ponds for a sufficient period to allow the fines to settle out of the water. It has been determined, however, that it takes an extremely long period of time for the fines to settle out, resulting in ever increasing tailings ponds. Additionally, water becomes trapped in the interstitial spacing between particles so that even after the fines have settled a large amount of water is trapped in the settled material. Other than the excessive water requirements, tailings ponds create an environmental and logistical challenge as tailings must be continually disposed of in the continuously growing volume of tailings ponds which must be contained and maintained for years. There thus exists a need for a method of processing oil sands that obviates the need for extensive tailings ponds and provides for the recycling of water from the tails soon after deposition at a deposition site.
A further limitation of the prior art is that there is no practical solution provided for handling tailings. Rather, current deposition methods result in a separation of a course tails and a fine tails, maintaining the need for extensive tailings ponds to provide settlement of the fine tailings component. There thus exists a need for a method of processing oil sands that produces a whole dry tails comprising both the sand component and the fine tailings.
There thus exists a need to increase the efficiency of excavation and transport processes to reduce operating costs. There exists an additional need to increase the operating period for an excavator servicing a transportable slurry facility, without increasing the distance of ore transport from the excavator to the facility. There exists a further need for a process capable of removing the mineral component of the oil sands at a proximate location to the mine face without the creation of extensive tailings ponds.